This invention relates in general to lasers, and in specific to methods for fabricating a broad spectrum emitter array and systems for using a broad spectrum emitter array.
Known incoherently beam combined (IBC) lasers combine the output from an array of gain elements or emitters (typically consisting of semiconductor material, such as GaAlAs, GaAs, InGaAs, InGaAsP, AlGaInAs, and/or the like, which is capable of lasing at particular wavelengths) into a single output beam that may be coupled into, for example, an optical fiber. The gain elements may be discrete devices or may be included on an integrated device. Due to the geometry of IBC lasers, each gain element tends to lase at a unique wavelength. Exemplary arrangements of IBC lasers are described in U.S. patent application Ser. No. 6,052,394 and U.S. patent application Ser. No. 6,192,062.
FIG. 1 depicts a prior art arrangement of components in IBC laser 10. IBC laser includes emitters 12-1 through 12-N associated with fully reflective surface 11. The optical power emitted by emitters 12-1 through 12-N is generated in their quantum wells (not shown) which are surrounded by waveguide layers (not shown) and cladding layers (not shown). The cladding layers confine the light produced by the laser in the waveguide layers and the gain region in a single mode. Semiconductor lasers that use quantum wells offer dramatically lower threshold current densities compared to bulk heterostructures and are therefore advantageous due to their higher efficiency.
In known IBC laser devices, each emitter is exactly the same, i.e., emitters 12-1 through 12-N are grown via a single fabrication process and, hence, possess identical characteristics. Moreover, each emitter in known IBC laser technology only possesses identical quantum wells in the active region of the respective emitter. Accordingly, the intrinsic bandwidth of each emitter is limited to the bandwidth of the identical quantum wells defined by the selected fabrication process.
Emitters 12-1 through 12-N are disposed in a substantially linear configuration that is perpendicular to the optical axis of collimator 15 (e.g., a lens). Collimator 15 causes the plurality of beams produced by emitters 12-1 through 12-N to be substantially collimated and spatially overlapped on a single spot on diffraction grating 16. Additionally, collimator 15 directs feedback from partially reflective 17 via diffraction grating 16 to emitters 12-1 through 12-N.
Diffraction grating 16 is disposed from collimator 15 at a distance approximately equal to the focal length of collimator 15. Furthermore, diffraction grating 16 is oriented to cause the output beams from emitters 12-1 through 12-N to be diffracted on the first order toward partially reflective component 17, thereby multiplexing the output beams. Partially reflective component 17 causes a portion of optical energy to be reflected. The reflected optical energy is redirected by diffraction grating 16 and collimator 15 to the respective emitters 12-1 through 12-N. Diffraction grating 16 angularly separates the reflected optical beams causing the same wavelengths generated by each emitter 12-1 through 12-N to return to each respective emitter 12-1 through 12-N. Accordingly, diffraction grating 16 is operable to demultiplex the reflected beams from reflective component 17.
It shall be appreciated that the geometry of external cavity 13 of IBC laser 10 defines the resonant wavelengths of emitters 12-1 through 12-N. The center wavelength (xcexi) of the wavelengths fed back to the ith emitter 12-i is given by the following equation: xcexi=A[sin(xcex1i) +sin(xcex2)]. In this equation, A is the spacing between rulings on diffraction grating 16, xcex1i is the angle of incidence of the light from the ith emitter on diffraction grating 16, and xcex2 is the output angle which is common to all emitters 12-1 through 12-N. The overall bandwidth of IBC laser 10 is xcex1-xcexN, or xcex94xcexlaser. As further examples, similar types of laser configurations are also discussed in U.S. patent application Ser. No. 6,208,679.
As previously discussed, in known IBC laser technology, each laser diode is the same as the others, and each quantum well in a particular device is the same as the other quantum wells of the device. The quantum wells provide a peak gain at a particular wavelength, xcexC, or center wavelength, and have a bandwidth of xcex94xcexQW. The quantum well bandwidth is the range of wavelengths over which the quantum wells can provide a gain. Thus, the laser array is constrained by the bandwidth of the quantum wells, such that the bandwidth of the laser array, xcex94xcexlaser, must be less than the bandwidth of the quantum wells, xcex94xcexQW.
Additionally, Raman amplifiers have been developed to amplify optical signals. A Raman amplifier relies upon the Raman scattering effect. The Raman scattering effect is a process in which light is frequency downshifted in a material. The frequency downshift results from a nonlinear interaction between light and the material. The difference in frequency between the input light and the frequency downshifted light is referred to as the Stokes shift which in silica fibers is of the order 13 THz.
When photons of two different wavelengths are present in an optical fiber, Raman scattering effect can be stimulated. This process is referred to as stimulated Raman scattering (SRS). In the SRS process, longer wavelength photons stimulate shorter wavelength photons to experience a Raman scattering event. The shorter wavelength photons are destroyed and longer wavelength photons, identical to the longer wavelength photons present initially, are created. The excess energy is released as an optical phonon (a lattice vibration). This process results in an increase in the number of longer wavelength photons and is referred to as Raman gain.
As is well understood in the art, SRS is useful for generating optical gain. Optical amplifiers based on Raman gain are viewed as promising technology for amplification of WDM and DWDM telecommunication signals transmitted on optical fibers. Until recently, Raman amplifiers have not attracted much commercial interest because significant optical gain requires approximately one watt of optical pump power. Lasers capable of producing these powers at the wavelengths appropriate for Raman amplifiers have only come into existence over the past few years. These advances have renewed interest in Raman amplifiers.
Single cavity IBC lasers have typically been considered inappropriate to stimulate Raman gain for many telecommunication networks, because known IBC laser technology suffers from limited bandwidth. Specifically, Raman amplifiers based on IBC laser technology will operate over a bandwidth that is limited by the intrinsic gain bandwidth (as defined by the quantum well characteristics) of the semi-conductor material from which the device is made. The intrinsic gain bandwidth is due to the limitations of the emitters used in the known IBC laser designs. Known amplifiers used in telecommunication networks typically have bandwidths of about 40 nanometers (nm) at the wavelengths of interest, namely the C (1530 to 1565 nm) or L (1570 to 1610 nm) bands. However, known IBC technology cannot generate gain over the entire wavelength range. In particular, known IBC laser technology is not sufficient for the current systems operating at both the C and L bands, and is unsatisfactory for future systems operating at the S (1430 to 1530 nm), C, L, and XL (1615 to 1660 nm) telecommunication bands.
The present invention is directed to systems and methods for generating Raman gain utilizing an IBC laser that possesses heterogeneous emitter structures. Specifically, an emitter array may be fabricated according to embodiments of the present invention such that the quantum well characteristics of respective emitter elements in the emitter array are tailored to permit efficient operation over different spectra. For example, in an embodiment of the present invention, emitters in an emitter array may be divided into two groups. Each emitter within the first group is substantially identical. Similarly, each emitter in the second group is substantially identical. The emitters in the first group are preferably implemented to possess a given bandwidth and a first center wavelength. The emitters in the second group are preferably implemented to possess a given bandwidth and a second center wavelength. The second center wavelength may be shorter than the first center wavelength. Accordingly, the emitter array may be oriented in an IBC laser such that the second group of emitters receives feedback that is associated with the blue portion of the feedback spectrum. Likewise, the emitter array may be oriented such that the first group of emitters receives feedback that is associated with the red portion of the feedback spectrum. By doing so, the intrinsic bandwidth of the IBC laser is increased and, hence, the IBC laser may generate Raman gain over a greater bandwidth that is suitable for typical telecommunication systems.
Embodiments of the present invention are directed toward fabrication methods for creating a suitable emitter array for an IBC laser that is used as a Raman pump. A suitable emitter array may be fabricated by growing the emitters on a single substrate. In the first stage of the fabrication process, the various layers (e.g. the confinement structure, the quantum well, the gain region, and/or the like) of the emitter array are grown on the substrate. The growth of the various layers may occur utilizing techniques that are known in the art. Additionally, in the first stage, the characteristics (e.g., quantum well composition and width) of the emitter of the emitters array are selected such that the emitters are designed to operate over a bandwidth centered at a first center wavelength. In a second stage of fabrication, a portion of the array is removed by, for example, suitable etching techniques. In a third stage of fabrication, a second set of emitters are regrown on the portion of the substrate where the other emitters were removed. The characteristics (e.g., quantum well composition and width) of the second set of emitters are selected such that the emitters are designed to operate over a bandwidth centered at a second center wavelength. Therefore, the emitter array (consisting of the two sets of emitters) may possess an appreciably increased intrinsic bandwidth by appropriately selecting the first and second center wavelengths.
Thus, embodiments of the invention increase the bandwidth of the laser array beyond xcex94xcexQW. By selecting different materials and/or a different thickness for the respective emitter groups of an emitter array, the laser array bandwidth, xcex94xcexlaser, can be increased to xcex94xcexQW+xcexCmax-xcexCmin, where xcexCmax is the maximum center wavelength of the emitter groups and xcexCmin is the minimum center wavelength for emitter groups.
Also, it shall be appreciated that embodiments of the present invention tend to provide similar output power from each emitter element in the array. Since embodiments of the present invention have multiple center wavelengths, which are dispersed across the bandwidth, embodiments of the present invention tend to provide power more evenly across the bandwidth.
It shall be appreciated that the present invention is not limited to any particular number of emitter groups in an emitter array. Embodiments of the present invention may grown, etch, and regrow any suitable number of emitter groups in an emitter array depending upon their intended use in a suitable application.
In alternative embodiments of the present invention, the emitters (of an array fabricated using the preceding technique) may be modified to individually possess greater bandwidth. In embodiments of the present invention, the emitters may possess multiple quantum wells. In other embodiments of the present invention, the quantum well thickness of the emitters may vary in a direction that is parallel with the emerging light, e.g. from front to back. By utilizing either of these techniques, the bandwidth of the individual emitters (and, hence, the emitter array also) may be augmented.